1. Field of the Invention
The present invention relates to an engine and, more particularly, to an engine with a continuously variable type valve drive device.
2. Description of the Related Art
JP Patent Application 2002-143037 discloses a valve drive device for an engine in which the opening period and lift amount of the intake and exhaust valves are continuously varied through a continuously variable valve drive device. In such a continuously variable valve drive device, in a boundary zone between a cam base circular part of a drive surface of a swing cam member and a cam nose part, a cam ramp is provided for a smooth transmission between both zones. The ramp height is determined based upon large lift conditions, which is used in high load operation. In such a configuration, in small lift conditions the ramp height is more than required. On the other hand, in a construction having a valve clearance, that is, a construction not including a lash adjuster for absorbing a valve clearance, generally the valve clearance is constant without depending on operation ranges of the engine. As a result, there is a problem that the small lift side has a ramp height more than required. Thus, the actual working angle becomes wider as an actual ramp height becomes larger, and thus it is difficult to realize a minimum working angle and a minimum lift required for a continuously variable type valve drive system.